Electrophotographic copiers of the image-transfer type, or plain-paper copiers as they are generally called, are well known in the art. In copiers of this type, an electrostatic latent image is first formed on a photoconductor by uniformly charging the photoconductor and then exposing the photoconductor to a light image of an original document to discharge portions of the photoconductor in a pattern corresponding to the graphic matter on the document. The photoconductor bearing the latent image is then subjected to the action of a developer, or toner, to form a developed toner image, which is transferred to a carrier sheet such as paper. Generally, in electrophotographic copiers employing the process described above, the photoconductor comprises an endless member, usually in the form of a drum, that is continuously moved at a predetermined velocity throughout the entire copy cycle. To transfer the developed toner image from the photoconductor to the carrier sheet, the sheet is brought into close proximity or actual contact with the photoconductor, while moving at the same velocity, in a transfer station.
In order to ensure that the leading edge of the carrier sheet is advanced to the transfer station in synchronism with the arrival of the leading edge of the developed toner image, the carrier sheet is first fed to a registration station, where it is momentarily held. As the leading edge of the developed image approaches the transfer station, feed members are actuated to advance the sheet from the registration station. By prefeeding the carrier sheet to the registration station in this manner, one avoids the loss of synchronism that may occur if the sheet slips relative to a feed member as it is initially fed from a stack.
Generally, in registration stations of the prior art, the leading edge of the carrier sheet is advanced to a registration position defined by a pair of opposing friction feed rollers, which remain stationary while the sheet is being held. One disadvantage of registration systems of this type is that the registration position depends on the sheet thickness, as well as the longitudinal extent of the registration nip. The longitudinal extent of the registration nip depends in turn on such factors as the compliance of the registration rollers and the normal nip force. Since these factors cannot be precisely controlled, the exact registration position of the carrier sheet remains uncertain.
Another problem encountered with registration systems of this type involves the acceleration of the carrier sheet to the photoconductor velocity when the registration rollers are actuated. Even a momentary slippage between the carrier sheet and the registration rollers will result in loss of synchronism between the leading edge of the sheet and that of the developed toner image. Further, if unequal slippage occurs across the width of the sheet, skewing will result. It is known in the art to advance a carrier sheet a sufficient distance from the stack so as to create a buckle in the registration nip, thereby urging the leading edge into the nip, so as to minimize slippage upon actuation of the registration rollers. However, even this expedient does not entirely eliminate the possibilities for slippage. Slippage is particularly likely to occur if the leading edge of the sheet is registered against a gate which is intermittently moved into the sheet path slightly upstream of the rollers.
Still another problem, inherent in image-transfer electrophotographic copiers generally, is that of separating the carrier sheet from the photoconductor surface following transfer of the developed image. A common expedient is to use a pickoff blade which intercepts the leading edge of the carrier sheet as it emerges from the transfer station to separate the adjacent edge portion of the sheet from the photoconductor. However, if such a blade is allowed to contact the photoconductor drum it will damage the drum surface over time. Further, the blade will become contaminated with remanent developer from the drum surface, producing streaks on the passing surface of the copy sheet.
It is known in the art, as shown in Hukuda et al U.S. Pat. No. 4,408,861, to deform a portion of the leading edge of the carrier sheet at the registration station so that the deformed edge portion remains spaced from the photoconductor at the transfer station. In such a system, the pickoff blade may be spaced slightly from the drum surface so as to avoid abrasion of the drum surface or contamination of the contacting blade portion. However, in the apparatus disclosed in the patent, all or a substantial portion of the leading edge of the sheet is bent away from the photoconductor, producing a corresponding void in the leading edge portion of the transferred image. Further, in the disclosed apparatus, in which a rotating deforming member urges the leading edge of the sheet against a resilient roller, the extent of sheet deformation remains uncertain.